Pellet stove

ABSTRACT

An improved burnpot assembly for a stove which allows clinker to be removed from the burnpot while simultaneously maintaining combustion includes a burnpot holder having an immovable floor, a burnpot movably supported on the burnpot holder, the burnpot including a front wall and an opposite back wall. The burnpot is movable with respect to the floor between a normal fuel burning position in which the walls of the burnpot and the immovable floor define a burnpot for combustion and a clinker removal position in which the burnpot is disposed laterally adjacent the immovable floor to allow clinker to drop from between the walls of the burnpot, such as into a clinker collection pan near the bottom of the stove.

FIELD OF THE INVENTION

This invention pertains to a heating apparatus and more particularly to a stove or furnace for generating heat from biomass fuels in the form of pellets or kernels.

BACKGROUND OF THE INVENTION

Due to the recent increase in the cost of natural gas and propane relative to the cost of biofuels, there has been a renewed interest in heating residential buildings with furnaces or stoves that burn biomass to generate heat. Biomass fuels or biofuels are generally defined as combustible organic materials obtained from recently living organisms or their metabolic byproducts. For residential heating applications, the most commonly used biofuels include corn and pellets prepared from plant materials, such as wood and/or agricultural chaff. In addition to being currently less expensive on a cost per unit energy basis than fossil fuel alternatives, biofuels are sometimes preferred because they are generally regarded as renewable sources of energy. It is also believed by some that because the carbon in biofuels was recently extracted from the atmosphere, burning of biofuels only returns the recently extracted carbon back into the atmosphere so that there is no net increase of carbon dioxide in the atmosphere, and hence there is no net increase in greenhouse gases that are believed by some to contribute to global warming.

Although corn and other biofuel burning residential heating apparatuses have been used for at least several decades, they have generally required a considerable amount of effort to maintain and use. Operators of such heating apparatuses were required to add fuel and remove clinker (a solid chunk of fused ash) on at least a daily basis. However, there have been substantial recent improvements in the way biofuel furnaces or stoves operate. Many commercially available biofuel furnaces have automated systems for transporting biofuel from a storage bin to the burner box where the fuel is combusted. These automated fuel delivery systems can add fuel at a rate determined by the heating demand, and may be regulated by a conventional thermostat.

There have also been several attempts to overcome the clinker removal problem as well. Clinker must typically be removed at least once per day, sometimes several times per day, during operation in cold winter climates, depending on the difference between the outside temperature and the desired inside temperature of the residential building.

U.S. patent application Publication No. 2005/0208445 A1 discloses a burnpot (also referred to as a burner box or firebox) having a movable floor and a translatable plate. The translatable plate is movable between a first position in which the entire interior of the burnpot is one continuous volume available for combusting fuel, and a second position in which the translatable plate divides the interior of the burnpot into two separate vertically adjacent volumes. The movable floor is movable between a first position in which it is capable of supporting burning fuel and accumulated ash, and a second position in which materials in the burnpot are allowed to drop downwardly into a combustible product tray (i.e., a clinker or ash pan). The removable floor and translatable plate allow a portion of the fused ash or clinker to be periodically removed from the burnpot and dropped into the clinker tray. After a clinker of sufficient size has formed at the bottom of the burnpot, the translatable plate cuts the ash column into two separate portions and divides the burnpot into an upper volume in which combustion is sustained and a lower volume consisting primarily of ash. Thereafter, the movable floor is moved aside to allow ash or clinker in the lower volume to drop into the tray below the burnpot. The movable floor is then moved back to its normal position for supporting combusting fuel, and the translatable plate in moved into its normal position causing the burning fuel which had been supported on the translatable plate to drop downwardly onto the movable floor. The movable floor and translatable plate allow clinker to be automatically removed on a prescribed or desired basis and deposited into a tray, thereby eliminating the need for daily or more frequent clinker removal, while maintaining combustion of fuel in the burnpot.

U.S. patent application Publication No. 2004/0200394 A1 discloses a corn burning stove having a burnpot with an openable bottom to allow ash to be removed from the burnpot, and a combustion retention slide for maintaining combustion while ash is being removed through the openable bottom of the burnpot. The combustion retention slide is positioned above the openable bottom at a location where fire burning from the openable bottom is at a level just beneath the location of the slide when it is moved into the burnpot. Fuel may then be deposited onto the combustion retention slide and ignited by the fire below. Thereafter, the openable bottom of the burnpot is opened to allow byproducts of combustion to drop downwardly into an ash pan. The openable bottom is then closed and the combustion retention slide is withdrawn from the burnpot allowing the burning fuel on the combustion retention slide to drop downwardly into the openable bottom. This arrangement allows clinker or ash to be removed as needed while combustion is maintained, and eliminates the need for daily or more frequent removal of clinker from the stove.

While the apparatuses described above are alleged to eliminate the need for daily removal of clinker by allowing clinker to be removed from the burnpot through an openable bottom or movable floor and deposited in an ash pan or tray located below the burnpot, they reduce or at least substantially impede the ability to allow the introduction of combustion air through the burning mass of fuel from underneath the burnpot due to accumulation of clinker in the space below the openable bottom or movable floor of these known apparatuses. The inability to effectively introduce combustion air to the burning fuel from underneath the fuel substantially reduces the ability to achieve complete and efficient combustion of the fuel, which results in substantial amounts of unburned or incompletely combusted materials being removed from the burnpot along with the ash or clinker. Incomplete combustion results in reduced efficiency and economy, and increased production of undesirable waste products.

SUMMARY OF THE INVENTION

The invention provides a burnpot assembly in which clinker or ash may be removed from a burnpot, while combustion of fuel is maintained in the burnpot, and in which clinker or ash removed from the burnpot is deposited in an ash pan that is laterally disposed of an airbox located underneath the burnpot.

In accordance with the invention, clinker or ash is removed from the bottom of a laterally movable burnpot by sliding sidewalls of the burnpot across a stationary or immovable floor of a burnpot assembly. This arrangement allows combustion air to be introduced upwardly into the burnpot and through the burning fuel from a cleanable airbox located underneath the burnpot, which does not accumulate any significant amount of clinker.

An additional advantage of removing clinker from the side rather than through the bottom of the burnpot is that clinker can be completely removed from the sidewalls of the burnpot by movement of the burnpot relative to stationary wedges that force clinker out of the burnpot and off of burnpot sidewalls.

Further, because clinker is removed from the side rather than through the bottom of the burnpot, it is possible to easily clean any small amount of ash that falls through the openings in the stationary burnpot floor by operation of an airbox scraper, without requiring clinker disposal. Instead, operation of the airbox scraper allows any ash in the airbox to be dumped into a clinker pan.

These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a pellet stove incorporating the burnpot assembly of the invention.

FIG. 2 is a perspective view of the arrangement of the various components of a burnpot assembly in accordance with an embodiment of the invention.

FIG. 3 is an elevational cross section of a burnpot assembly in accordance with the invention, showing the position of a movable burnpot and fuel retention plate during normal operation.

FIG. 4 is an enlarged elevational cross section showing the position of the fuel retention plate over the burning fuel in the burnpot immediately prior to clinker removal.

FIG. 5 is an elevational cross section showing the position of the movable burnpot in the burnpot assembly during clinker/ash removal.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A pellet stove 10 (FIG. 1) in accordance with the invention includes a burnpot assembly 12 (FIG. 2) comprising a burnpot holder 14 having an immovable floor 16, a movable burnpot 18, and a combustible fuel retention plate 20. The burnpot 18 and burnpot holder 14 are configured to allow the burnpot to be moved from a normal fuel burning position to a clinker removal position in which the burnpot is located laterally adjacent the normal fuel burning position. Defined by burnpot holder 14, adjacent immovable burnpot floor 16, is an opening 22 that allows clinker and/or ash to drop through into clinker pan 23 located below opening 22.

While not constituting an essential part of the invention, pellet stove 10 will preferably include heat exchange means for extracting heat from the product gases of combustion and transferring the heat to air that is circulated in a building to heat living space within the building. Conventional furnace heat exchangers, fans, blowers and the like may be employed for this purpose.

In the illustrated embodiment, burnpot 18 has a front wall 24, a back wall 26, and two opposite sidewalls 28, 30 connected at their respective opposite edges to the front wall and to the back wall to define a rectangular enclosure, which together with immovable burnpot floor 16 define a volume for burning fuel when the burnpot is in its normal fuel burning position. However, it is conceivable that full sidewalls 28, 30 could be replaced with other types of connecting members that allow the front wall 24 and back wall 28 to be moved together as a unit, with burnpot holder 14 being configured to define sidewalls of the burnpot.

In the illustrated embodiment, burnpot holder 14 defines an airbox 32 or air plenum located directly under immovable burnpot floor 16. Immovable burnpot floor 16 may be provided with a plurality of apertures 33 that are sufficiently small to prevent substantial amounts of ash, clinker or fuel to drop through into the airbox, but are sufficiently large to allow combustion air to freely enter burnpot 18. A blower (not shown) may be used to introduce air through conduit (not shown) into airbox 32 at a pressure slightly above ambient pressure.

Burnpot holder 14 may also be configured to define a plenum that surrounds the burnpot on three of its sides. In the illustrated embodiment, outer walls 34, 36 and 38 of burnpot holder 14, together with inner walls 40, 42 and 44 of burnpot holder 14 define an air plenum or plenums for introducing combustion air to fuel contained in burnpot 18. Inner walls 40, 42 and 44 have apertures 46 that allow combustion air to flow toward burnpot 18, and burnpot sidewalls 28, 30 have apertures 48 that allow combustion air to flow into burnpot 18 and contact fuel contained within the burnpot.

Burnpot holder 14 may also be configured to allow movement of the combustible fuel retention plate 20 from a position in which it is located laterally adjacent to burnpot 18 during normal operation (FIG. 3), to a position in which retention plate 20 is disposed over immovable burnpot floor 16 for clinker removal. Combustible fuel retention plate 20 may be slidably movable with respect to burnpot holder 14 between the normal fuel combustion position and the clinker removal position. Plate 20 is preferably located at an elevation immediately above the top of the fuel burning in burnpot 18 and includes a plurality of apertures 49 that allow flames from the fuel in the burnpot to ignite fuel desposited on the plate.

Employing burnpot assembly 12 of this invention, clinker may be removed from the burnpot while combustion of fuel is sustained on fuel retention plate 20. In accordance with the invention, after a clinker is formed or a substantial amount of ash has accumulated in burnpot 18, plate 20 is moved from a position laterally adjacent burnpot 18 and immovable floor 16, to a position (shown in FIG. 4) in which plate 20 is disposed directly above immovable floor 16 and above ash, clinker and burning fuel supported on floor 16. Thereafter, fuel 100 is deposited onto plate 20 where it is ignited by flames from the fuel burning below. After the fuel on plate 20 starts burning, and the fire is capable of at least temporarily sustaining itself during the clinker removal operation, movable burnpot 18 is moved from its normal position to a position (shown in FIG. 5) adjacent movable floor 16 and over opening 22. As burnpot 18 is moved toward opening 22, ash, clinker and other materials residing on immovable floor 16 are scraped or raked by front wall 24 toward opening 22. These materials are urged downwardly by wedges 92 projecting from rear wall 94 of burnpot assembly 12 causing clinker and any other materials in burnpot 18 to fall through opening 22 and be received in clinker pan 23. Slots 96 are provided in rear wall 26 of burnpot 18 to allow wedges 92 to enter burnpot 18 as it is moved into the clinker discharge position. The ash pan can be of a sufficiently large volume to accommodate clinker from several days of operation (e.g., about one week). The illustrated embodiment employs three wedges 92, including two wedges that scrape the inner surfaces of sidewalls 28, 30, and a third wedge 92 that is located between the sidewall scraping wedges.

After the ash and/or clinker falls through opening 22, movable burnpot 18 is moved back into its normal position directly over immovable floor 16. Thereafter, plate 20 is moved to its normal position laterally adjacent burnpot 18. A scraper or wall 50 scrapes burning fuel off of plate 20 as it is moved to its normal position, causing the burning fuel to drop down onto immovable floor 16 and into movable burnpot 18. Additional fuel is added as needed to sustain combustion in the burnpot 18 and to provide heat as required.

As shown in the illustrated embodiment, plate 20 is provided with a plurality of upwardly projecting posts 52 along a front edge 54 thereof. Posts 52 are not essential, but are useful for helping to more uniformly distribute or disperse burning fuel from plate 20 over the area of immovable floor 16.

In accordance with a preferred aspect of the illustrated embodiment, an airbox scraper or rake 56 is provided to remove ash particles that have fallen through apertures 33 of immovable floor 16. Rake 56 has an edge or blade 58 that scrapes floor 60 of airbox 32 as it moves from a back edge of the airbox toward the front of stove 10 causing ash that has accumulated on floor 60 to be removed from airbox 32 and deposited into clinker pan 23.

While it is conceivable that movable burnpot 18, plate 20 and rake 56 could be mechanically linked to manually operated levers, rods or the like, it is preferred that burnpot 18 and plate 20 are connected by mechanical linkages 64 and 66 to electric motors 70 and 72 operated by an electronic controller 80 (e.g., a microprocessor), whereby a homeowner using pellet stove 10 to heat a home is not required to perform any steps on a daily basis to effect clinker removal. Instead, clinker removal is fully automated so that the homeowner need only empty an ash pan, for example, on a weekly basis.

Rake 56 may also be mechanically linked via linkage 68 to an electric motor 76 to automatically clean the airbox of accumulated ash. However, because airbox cleaning is only required on an occasional basis (e.g., once per week), in the illustrated embodiments of rake 56 includes a lip 98 that extends out the front of stove 10 to allow rake 56 to be manually pulled forward to scrape ash from floor 60. In order to collect the ash from airbox 32 in clinker pan 23, door 99 of stove 10 is opened and clinker pan 23 is moved forward slightly before rake 56 is manually moved forward. For automated airbox cleaning, rake 56 need not include a lip and does not need to be moved forwardly to clean airbox 32, but instead can be moved rearwardly so that ash is automatically deposited in clinker pan 23 on a scheduled basis.

As is known in the art, fuel is preferably delivered to stove 10 automatically, such as with an auger or other fuel feeding mechanism. It has been determined that the amount by mass or weight of a pellet or kernel fuel delivered to the burnpot can be relatively accurately determined, such as by correlating the mass or weight feed rate to the number of auger turns. It has also been determined that corn kernels and other types of pellet fuels made for pellet stoves burn at a relatively consistent rate when combustion air supply is consistent so that the amount of time that it takes to burn a given amount of fuel in pellet stove 10 can be reliably determined. In such case, clinker removal operations involving movement of plate 20 and burnpot 18 can be automatically performed by programming the electronic controller to operate the motor(s) mechanically linked to plate 20 and burnpot 18 on a timed basis. Accordingly, use of automated clinker removal and automated fuel delivery regulated by a conventional thermostat will allow pellet stove 10 in accordance with the invention to be operated for several days (e.g., about one week) without any operator intervention. On an occasional basis (e.g., from a few days to about one week), it will be necessary to refill a fuel hopper (not shown), remove clinker that has been automatically removed from the burnpot and deposited in an ash pan, and possibly perform some minor cleaning operations.

The stove 10 may be provided with a controller that operates a combustion air fan and a fuel delivery mechanism that together regulate the rate at which fuel and air are supplied and thus the amount of heat released by combustion in the burnpot. The controller can be programmed to operate the stove in a pilot mode in which fuel and air are provided at rates that are sufficient to maintain combustion but not produce a substantial amount of heat when heating of a building is not desired. The controller may also be programmed to provide increasing amounts of fuel and combustion air during start-up in incremental steps to prevent excessive fuel additions at start-up from smothering the fire in the burnpot.

The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents. 

1. A burnpot assembly for a stove, comprising: a burnpot holder having an immovable floor; a burnpot movably supported on the burnpot holder, the burnpot including a front wall and an opposite back wall, the front wall and back wall connected together by members that allow the opposite front wall and back wall to be moved together as a unit with respect to the burnpot holder between a normal fuel burning position in which burning fuel may be deposited on the immovable floor between the opposite sidewalls, and a clinker removal position in which the burnpot is disposed laterally adjacent the immovable floor.
 2. The burnpot assembly of claim 1, wherein an opening is defined in the burnpot holder to allow clinker to drop through from the burnpot when it is moved from the normal fuel burning position to the clinker removal position.
 3. The burnpot assembly of claim 1, wherein the members connecting the opposite front wall and back wall are sidewalls.
 4. The burnpot assembly of claim 1, in which an air plenum in defined below the immovable floor of the burnpot holder, and the immovable floor has apertures that allow air to flow from the air plenum into a space defined above the immovable floor.
 5. The burnpot assembly of claim 1, in which an air plenum is defined by the burnpot holder adjacent at least a first wall of the burnpot, the plenum including a wall adjacent the first wall, the plenum wall and the adjacent burnpot wall each having apertures to allow air to flow from the plenum into the burnpot.
 6. The burnpot assembly of claim 1, further comprising a combustible fuel retention plate movably supported on the burnpot holder, the fuel retention plate being movable between a first position in which the fuel retention plate is located laterally adjacent the burnpot and a second position in which the fuel retention plate is disposed above the immovable floor of the burnpot holder.
 7. The burnpot assembly of claim 1, in which the burnpot is mechanically linked to a motor operated by an electronic controller.
 8. The burnpot assembly of claim 1, in which the fuel retention plate is mechanically linked to a motor operated by an electronic controller.
 9. The burnpot assembly of claim 4, in which the air plenum below the immovable floor contains a scraper for removing ash particles from the air plenum.
 10. The burnpot assembly of claim 9, in which the scraper is mechanically linked to a motor operated by an electronic controller.
 11. The burnpot assembly of claim 1, further comprising wedges that project from a rear wall of the burnpot assembly and are received through slots in a rear wall of the burnpot as it is moved to the clinker removal position, the wedges configure to engage clinker in the burnpot as the burnpot is moved to the clinker removal position.
 12. A process for removing clinker from a burnpot of a stove, comprising: adding fuel to a burnpot defined by burnpot walls and a floor, the walls being connected together and being movable with respect to the floor; burning fuel and forming clinker in the burnpot; moving the walls of the burnpot with respect to the floor and thereby causing the clinker to be removed from the floor; dropping the clinker from between the walls of the burnpot; moving the walls of the burnpot back into a position in which the burnpot walls and the floor together define a burnpot.
 13. The process of claim 12, further comprising positioning a combustion retention plate over the burning fuel, depositing fuel on the combustion retention plate and allowing the fuel on the combustion retention plate to be ignited by flames from the fuel burning below the combustion retention plate, and thereafter performing the step of moving the walls of the burnpot with respect to the floor, whereby combustion is maintained during clinker removal.
 14. The process of claim 13, further comprising removing the combustion retention plate from above the burnpot and depositing fuel burning on the combustion retention plate into the burnpot after moving the walls of the burnpot back into a position in which the burnpot walls and the floor define a burnpot. 